Positron emission tomography (PET) using a labeling agent exhibiting excellence in sensitivity and quantitative performance has widely been used in clinical practice in recent years in order to obtain information that can be reflected in a treatment plan such as cancer detection, determination of benignity versus malignancy, determination of the effect of treatment, diagnosis of relapse, and prognostic prediction. Widespread use of PET testing develops a fear of exposure of examinees and healthcare workers (doctors, radiographers and nurses) to radiation. Controlling the exposure to a minimum possible level is an important problem to be solved. Meanwhile, in order to ease examinee's anxiety during testing, it is desired that a radiation diagnostic device with high resolution and high detection efficiency capable of making a determination promptly and appropriately be offered at low cost.
An example of a gamma ray detector used in such a radiation diagnostic device is disclosed in Patent Document 1. In the gamma ray detector disclosed therein, a collimator, NaI (sodium iodide), and a PMT (photomultiplier tube) are stacked. Parallel holes of a small diameter are defined in the lead collimator, and only those gamma rays that travel from a direction of the holes are guided by the collimator to a single crystal of NaI. Then, fluorescent light of an intensity proportionate to the energy transfer from the gamma rays is generated at the single crystal. The fluorescent light is detected by the PMT, so that the intensity and position of the gamma rays are determined.
Patent Document 2 discloses a dual-purpose device functioning both for SPECT (single photon emission computed tomography) and PET where gamma ray detectors are arranged to place a target of measurement therebetween.
Further, Patent Document 3 discloses a gamma ray detector where a group of scintillators with a large number of columnar scintillators closely connected together, and an APD array with a large number of avalanche photodiodes (APD) closely connected to the scintillators, are optically coupled.    [Patent Document 1] Japanese Patent Application Laid-Open No. Hei. 5-66275    [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-159682    [Patent Document 3] Japanese Patent Application Laid-Open No. Hei. 7-311270
However, there are problems that use of a collimator as in the technique disclosed in Patent Document 1 increases the weight and size of a detector. The technique disclosed in Patent Document 2 finds it difficult to accurately specify the position of a radionuclide. Further, the technique disclosed in Patent Document 3 requires a radiation detector of a complicated structure.